Alkaline storage batteries are repeatedly rechargeable and have been used for power sources for a number of portable appliances. In particular, nickel-metal hydride storage batteries have been in broad use as having a high energy density and being environmentally friendly.
Positive electrodes for an alkaline storage battery comprise a conductive base material, an active material and a binder.
For the conductive base material used has been a foamed metal sheet, a punched metal sheet, an expanded metal sheet or the like.
For the active material, for example, a powder comprising nickel hydroxide, a powder comprising nickel hydroxide with the surface thereof coated with cobalt hydroxide, or a powder comprising nickel hydroxide with the surface thereof coated with cobalt oxyhydroxide has been generally in use for the alkaline storage battery.
For the binder used has been a fluorine resin such as polytetrafluoroethylene (hereinafter referred to as PTFE) as proposed in Japanese Laid-Open Patent Publication No. Hei 11-25962.
The binder is used for preventing the active material from falling off or separating from the base material and for binding active material particles to each other.
There are problems lying in the above-mentioned binder comprising a fluorine resin as follows:
(1) There is a case that, in the process of applying a paste comprising the active material and a fluorine resin as the binder to a conductive base material, the fluorine resin in the paste becomes fibrous depending on shear strength or temperature conditions. For example, when PTFE becomes fibrous, the viscosity of the paste increases and the flowability decreases, causing instability of the rheological property thereof. There is also a case that, in rolling process to extend the conductive base material applied with the paste, the base material is cut off by the lump of the fiber.
(2) Since the conventional binder comprising a fluorine resin is weakly bound to a metal constituting the conductive base material, the active material layer, which is obtained by applying the paste to the base material followed by drying, has a high tendency to fall off the base material. For this reason, a battery capacity decreases during discharging at a large current.
(3) Since the conventional binder comprising a fluorine resin has insufficient flexibility, the active material layer tends to have a crack therein when the electrode plate using the binder is wound up. As the cracking leads to the fall of the active material layer off the base material, there is no obtaining a sufficient battery capacity. A charge and discharge reaction also leads to the fall of the active material layer off the base material and, therefore, the repeated charge and discharge of the battery causes a decrease in the battery capacity.